Many applications for LED (light-emitting diode) based solid-state lighting (SSL) require dimming capabilities. LEDs react instantaneously to changes in power input, making SSL especially appropriate for dimming scenarios. However, LED luminaire designers face a challenge in designing products that work with a variety of legacy dimming-control technologies and in some cases that offer the ability to operate in emerging wireless-network-control scenarios. Luminaire designers must first understand the related but separate concepts of the mechanism used to feed the dimming information into the luminaire, and the technique used to actually reduce the brightness of the LEDs. There are two basic alternatives that can be used to reduce the light output of the LEDs: analog dimming and pulse-width-modulation (PWM) dimming.
Analog dimming simply controls the drive current fed to the LEDs. Full brightness uses the full current. The driver electronics linearly reduces the current to dim the LEDs. Analog dimming can be simple to implement, but may not deliver the best overall performance. The efficiency of LEDs tends to increase at lower currents, but LEDs may not produce a consistent color at lower drive currents.
For PWM dimming, the driver electronics supplies pulses of full-amplitude current to the LEDs. The driver varies the duty cycle of the pulses to control the apparent brightness. PWM dimming relies on the capability of the human eye to integrate the average amount of light in the pulses. Provided the pulse rate is high enough (typically about 200 Hz), the eye does not perceive the pulsing but only the overall average.
For an LED luminaire to respond correctly to a phase-control dimmer, it is necessary to add several functional blocks into the driver electronics. For example, a sensor is typically provided for monitoring the AC input waveform before the power-factor-correction (PFC) stage and generating an output signal proportional to the amount of phase cut. The controller drives the MOSFET switch connected at the input of a DC-DC converter. When no AC phase-cut is detected, the output is driven at 100% duty cycle to give full brightness. AC phase-cutting reduces the lamp brightness by cutting out part of the AC waveform. For example, a forward-phase triac dimmer cuts out a portion of the AC waveform at the leading edge of each half sine wave. In contrast, a reverse-phase dimmer cuts out a portion of the AC waveform at the trailing edge so that the dimmer turns off part-way through the sine wave. In either case, LED dimming is achieved by cutting out part of the AC waveform. It is desirable to reduce the number of components needed to support LED dimming, thereby simplifying the LED driver design and reducing system cost.